The invention relates to a method for managing the state of charge of a traction battery of a rechargeable hybrid vehicle, comprising a hybrid power train of the type comprising a combustion engine disposed for propulsion and an electric motor/generator disposed for propulsion.
Rechargeable hybrid vehicle is the term used when a hybrid vehicle, having two independent energy sources, can be recharged on the electrical grid, which makes it possible to have a greater autonomy in electric mode than a conventional hybrid vehicle. In fact, the traction battery, which powers the electric motor, is conventionally recharged by the combustion engine when running, or by using the kinetic energy of the vehicle, and an additional option then consists of being able to recharge the battery by connecting it to the electrical grid.
In the context of combustion/electric hybrid motorization, the “Plug-in Hybrid Electric Vehicle” (PHEV) technology is known in particular, in which the two motorizations act together to propel the vehicle. The use of the electric mode will depend on the configuration of the vehicle. Two main operating modes exist in particular:
In an operating mode known as “Charge Depleting” (CD), corresponding to a “Zero Emission Vehicle” (ZEV) operating mode, the battery supplies substantially all or virtually all of the power necessary for propulsion. In an operating mode described as “Charge Sustaining” (CS), the battery supplies only a complement of power, the essential part of the power necessary for propelling the vehicle then being supplied by the combustion engine. The latter mode corresponds to an operating mode similar to the “Hybrid Electric Vehicle” (HEV) mode.
Depending on the power setting intended to be transmitted to the driving wheels of the vehicle and on the energy management law implemented by the energy management system of the vehicle, typically a high performance computer, for example a general supervisor of the power train, the vehicle will operate either in CD mode or in CS mode.
Conventionally, as illustrated in FIG. 1, in the case where the driving phase starts with the battery having a high state of charge, for example, at full charge SOC_Max, the vehicle is used in CD operating mode and the energy management system will allow the state of charge of the battery to drift in this operating mode until a predefined minimum state of charge value SOC_Min_CD is reached. Once this condition is reached, the driver has the possibility of being able to continue to drive in CS mode, in which the state of charge of the battery is kept constantly around this predefined minimum value.
As a variant, as illustrated in FIG. 2, the driver can also impose a CS operating mode, even when the state of charge of the battery remains at a high value, around which the state of charge of the battery will be kept in CS mode, in order to save the CD mode for the end of the journey, for example for arriving in town. However, as in the operation illustrated in FIG. 1, when the state of charge of the battery will have reached the predefined minimum value SOC_Min_CD in CD mode, the energy management system of the vehicle will automatically control the switchover to CS mode.
It appears that the utilization range of the battery, that is to say, the state of charge range over which the driver can use the battery, both charging and discharging, is an important parameter taken into account by the energy management law that allows the operating mode of the vehicle to be defined.
The utilization range of the battery is characterized in particular by the permitted maximum state of charge of the battery, that is to say, the maximum state of charge beyond which the battery is not permitted to rise. Too high a permitted maximum state of charge encourages rapid deterioration of the state of health of the battery, whereas too low a permitted maximum state of charge causes detriment in terms of energy available to the driver, in particular in CD mode. A compromise must therefore be found, through suitable management of the end of charge voltage, between the energy it is desired to provide in the short term and the durability of the battery.
The utilization range of the battery is also characterized by the permitted minimum state of charge of the battery, that is to say, the minimum state of charge below which the battery is not permitted to fall. Too high a permitted minimum state of charge is detrimental to the energy available to the driver, in particular in CD mode, whereas too low a permitted minimum state of charge is a problem in so far as there is a risk that the battery will not be able to supply the required minimum power value, in particular in CS mode. A compromise must therefore also be found, through management of the permitted minimum state of charge, between the energy it is desired to provide to the driver (that is to say, the autonomy) and keeping an acceptable level of performance, in particular in terms of minimum available power being discharged from the battery, both in CD mode and in CS mode.
Also, one of the major challenges relates to the durability and to the preservation of the performance of the battery over time. In this respect, the battery suffers two types of ageing. On one hand, ageing known as calendar ageing, which is deterioration of performance linked to the time spent at different states of charge and of temperature, and, on the other, ageing by cycling, which is deterioration linked to the power values that have passed through the battery. Now, today, in particular if it were necessary to be able to have the battery operating in CD mode for the entire life of the battery, it is very difficult to ensure the durability of the battery on account of these ageing constraints. In extreme cases, the battery can go into “sudden death”, which is characterized by a drastic increase of the internal resistance of the battery cells and a reduction of its useful capacity.
A method is known, from the patent document FR3002045, for managing the utilization range of a battery depending on the ageing of the latter, which consists, in particular, of estimating the end of charging voltage depending on the state of health in energy of the battery, so as to increase the permitted maximum state of charge and consequently, the useful state of charge range, depending on the ageing state of the battery. Such a method makes it possible to ensure a minimum required energy value, while limiting the deterioration of the battery. However, the durability of the battery still remains limited.
In this context, a need therefore exists for a method for energy management of a battery, exempt from the abovementioned limitations, and, in particular, which makes it possible to preserve the durability of the battery.